Because without them, people do things they normally wouldn’t do. In the case of political opinions, it seems that conservatives get more conservative and liberals get more liberal. This is according to Cass Sunstein et al., who blogged about groups and extremism the other day.
Groups have always fascinated me. I guess if you really want want people’s unbiased opinions about what to do (“should we roll out the new server today or wait another week?”), you need to have some diversity in the crowd. Otherwise, people won’t tell you what they really believed. Or says one interpretation of Sunstein’s data (mine, not necessarily his).